Small bodies · Space Codex
Kuiper belt
A broad doughnut of icy bodies beyond Neptune's orbit, home to Pluto, Arrokoth and many short-period comets.
The icy disc beyond Neptune
At a glance
Key points
- Extends from about 30 to 50 AU
- Home to Pluto and Arrokoth
- A source of short-period comets
Relics of formation
Kuiper belt objects are leftovers from the outer disc that built the giant planets. Neptune's migration scattered many of them; some were flung inward to become short-period comets, others outward to the Oort cloud.
How it was found
Pluto was spotted in 1930, but more than sixty years passed before anything else was seen beyond Neptune. In August 1992 David Jewitt and Jane Luu, using the University of Hawaii's 2.2 m telescope on Mauna Kea, found a faint body now catalogued as 1992 QB1. It confirmed that Pluto was not alone but one member of a vast population, and thousands more have been found since.
Three kinds of orbit
The belt is several populations, not one. 'Cold classical' objects such as Arrokoth follow nearly circular, barely tilted paths that seem undisturbed since they formed. Resonant objects are locked in step with Neptune; Pluto circles the Sun twice for every three Neptune orbits, which keeps the two from ever colliding. Beyond them lies the scattered disc, bodies flung by Neptune onto stretched, steeply tilted orbits reaching to nearly 1,000 AU.
Not much mass
For all its extent, the belt holds no more than about a tenth of Earth's mass. Models suggest the region once held 7 to 10 Earth masses of material, most of it later scattered away as the giant planets shifted their orbits.
What we still don't know
How many large bodies remain undiscovered in the belt's outer reaches. A deep survey made for New Horizons with Japan's Subaru Telescope reported in 2024 an unexpected excess of faint candidates beyond about 70 AU. If their orbits are confirmed, they could mark a second, more distant belt; for now they remain unconfirmed.
Missions
- 2006–
Pluto flyby 14 Jul 2015; Arrokoth 1 Jan 2019; now studying the outer heliosphere.
Related entries
- PlutoDwarf planet · heart of the Kuiper belt
- 486958 ArrokothThe most distant world explored up close
- Oort cloudThe Solar System's distant icy shell
Sources
- Fraser et al. 2024, candidate distant TNOs, Planetary Science Journal (opens in a new tab)
- NASA Science: Kuiper belt facts (opens in a new tab)
- NASA Science: New Horizons (opens in a new tab)
- IAU Circular 5611 (1992), via D. Jewitt, UCLA (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.